A conventional solar cell, one of the use of an organic photoelectric conversion device, has been developed and practically used which device is made of polycrystal silicon. The production of the device requires high purity silicon and a high-temperature process. Considering the energy required for production, the contribution to energy saving technology of solar cell has been questionable. Besides outdoor electric generation use, the conventional production method still has a problem in fabrication of the device on a plastic substrate required for portable solar cells.
Another usage of an organic photoelectric conversion device as an optical sensor is represented by an image sensor incorporated in a facsimile or a photocopier. Such an optical sensor has been put into practice in an image reader with a linear sensor, serving as a scanner, made of silicon crystal. However, there has not hitherto been a two-dimensional scanner developed for practical use, which is large in area and requires no scanning.
In order to solve the above problems, efforts have been made to develop a solar cell which can be produced with less energy and which can be formed by a simple coating method with organic material which method can enable formation of a layer large in area.
For example, it has been proposed that organic material that has undergone dye sensitizing is used for the production of a wet solar cell. However, since the wet solar cell uses an electrolytic solution in the structure of the cell and may cause solution leak and iodine dissipation in the solution, such a cell has not been put into practice yet.
Another solar cell formed of organic material is exemplified by an organic thin layer solar cell entire of which is solid.
A solar cell (an organic solar cell) formed of an organic semiconductor may have various structures such as a dye sensitizing type, a bulkhetero junction type, a hetero pn junction type, and a Shottky type, for example (see Patent reference 1, and Non-Patent references 1 and 2).
A hetero junction type is a formation fabricated by depositing a layer made of an electron donor and a layer made of an electron acceptor, and uses charge transport caused by photoinduction at the interface of the layers. A reported example of the solar cell was formed of copper phthalocyanine and a perylene derivative respectively for an electron donor and an electron acceptor and attained a conversion efficiency of 1% in Non-Patent reference 3. Alternatively, a condensed aromatic hydrocarbon, such as pentacene and tetracene, can be proposed for an electron donor, and a fullerene compound such as C60 is proposed as an electron acceptor.
A bulkhetero junction type fabricates an active layer by mixing an electron donor and an electron acceptor at an appropriate ratio, and is therefore different from a hetero junction type that has an active layer in a two-layer structure. Junctions of the electron donor and the electron acceptor are uniformly spread in the bulk of the mixture active layer, so that solar light can be efficiently utilized. A device with a bulkhetero junction can be produced by co-depositing an electron donor and an electron acceptor by vacuum deposition to form an active layer or by coating, such as spin coating or printing, with the mixture solution of both electron donor and acceptor. An active layer has been reported which has been fabricated by vacuum deposition using copper phthalocyanine and C60 (Non-Patent reference 4). A wet coating method typically uses a mixture of conjugated polymer of polythiophene and [6,6]-phenyl C61-butyric acid methylester (abbreviated to PCBM) which is a soluble derivative of fullerene (Non-Patent reference 5).
Reported production methods of the above solar cell of a bulkhetero junction type fabricate a layer by coating with a mixture of a polythiophene derivative or a poly phenylene vinylene derivative and a fullerene (C60) derivative (see Patent reference 2 and Non-Patent reference 6). The reports mentioned that the mixture layer of a solar cell thus produced has conjugated polymer and a fullerene compound that are separated in phase from each other.
It has been reported that a solar cell formed of a benzoporphyrin compound takes the form of a device with a Schottky junction (Non-Patent reference 7) or takes the form of a device with a hetero junction in which the electron acceptor layer has been formed of a perylene derivative (Non-Patent Reference 8 and Patent Reference 3). Both of the above devices however have low conversion efficiency and therefore large problems remain to be unsolved for practical use.
In the meanwhile, an organic pigment as represented by phthalocyanine, benzoporphyrin, quinacridone, and pyrrolopyrrole, is known to the public because the pigment can serve as high crystalline pigment material and has high durability under irradiation with light.    [Patent reference 1] Japanese Patent Application Laid-Open (KOKAI) No. HEI 8-500701    [Patent reference 2] Japanese Patent Application Laid-Open (KOKAI) No. HEI 6-179802    [Patent reference 3] Japanese Patent Application Laid-Open (KOKAI) No. 2003-304014    [Non-Patent reference 1] “State-of-the-art organic thin layer solar cell”, November, 2005, CMC Publishing Co., Ltd.    [Non-Patent reference 2] “dye sensitization solar cell—latest technology and market”, July, 2004, TORAY RESEARCH CENTER. Inc.    [Non-Patent reference 3] C. W. Tang, “Appl. Phys. Lett.”, vol. 48. pp. 183-185, 1986    [Non-Patent reference 4] S. Uchida et al., “Appl. Phys. Lett”, vol. 84, pp. 4218-4220, 2004    [Non-Patent reference 5] S. E. Shaneen et al., “Appl. Phys. Lett”, vol. 78, pp. 841-843, 2001    [Non-Patent reference 6] “Material Research Society Bulletin” (sic), vol. 30, No. 1, 33, 2005    [Non-Patent reference 7] K. Yamashita et al., “Bull. Chem. Soc. Jpn”. Vol. 60, pp. 803-805, 1987    [Non-Patent reference 8] D. Wohrle et al., “J. Mater. Chem.”, vol. 5, pp. 1819-1829, 1995